Hi Guys:
Highly unlikely that it would be an artillery range finder. Range finders were normally not issued to field artillery detachment commanders (Sgt in charge of the gun). The Detachment (artillery does not have "crews") Commanders were (and still are) trained to estimate ranges if they had to engage a target with direct fire.
The normal indirect fire mode requires recording all the guns of a battery onto a common orientation by using a survey instrument to lay each sight onto a distant aiming mark, a pair of aiming posts, a paralleloscope or a collimeter. The sights of a field artillery piece are in fact a very fine survey instrument not a telescopic sight as per a sniper rifle. Guns are almost never moved with thier sights fitted. As each sight has to be carefully calibrated to the gun, they are far too valuable to risk damaging. They are carried in a padded sight box and are fitted to their mount as part of the "coming into action" drills.
Another thread has a discussion about the paralleloscope.
Anti-aircraft Detachment Commanders might be issued a small range finder but, again, in the short time an aircraft is exposed during an engagement, it is faster to estimate the range and correct by observing where the tracer goes. Firing the Bofors while hooked to the tractor is possible but is such an unusual situation that it would seem odd to put a bracket on the roof just to hold the range finder, a valuable and fragile piece of kit that would be normally carried in a padded sight box.
Having spent a good portion of my life riding in gun tractors (both Her Majesty's and my own), I would vote for the SMG as I would want my personal weapon easy to hand; not stuck behind the seat or bouncing around on the floor while I'm poked up out of the roof hatch. A more detailed photo would be handy.
Mike (Instructor-in-Gunnery, Royal Canadian Artillery)